I. Field of the Invention
The present invention relates to magnetic resonance imaging (MRI) and spectroscopy devices and more particularly, to a feedback circuit for a magnetic resonance imaging or spectroscopy device having an MRI receiver antenna, a resonant circuit for generating output voltages in response to nuclear magnetic resonance magnetization signals received by the MRI receiver antenna and an amplifier for amplifying the output voltages.
II. Description of the Related Art
In magnetic resonance imaging and magnetic resonance spectroscopy devices, time dependent nuclear magnetic resonance magnetization signals are detected and amplified to output a signal proportional to the time derivative of the magnetization signals. The signals proportional to the time derivative of the magnetization signals are processed to obtain a cross sectional or three-dimensional image of an object such as a patient or are analyzed to obtain the frequency spectrum of the magnetization signals to obtain information about the chemistry of the object. The nuclear magnetic resonance magnetization signals are weak, and accordingly, the signal to noise (S/N) ratio must be good.
A conventional circuit for a magnetic resonance imaging or spectroscopy device includes a resonant circuit and an amplifier. The resonant circuit includes an MRI receiver antenna which may be represented by an inductor having an inductance L, a capacitor having a capacitance C.sub.t and a parallel resistor with a resistance R.sub.p representing all the dissipations of the resonant circuit. The MRI receiver antenna L, capacitance C.sub.t and parallel resistance R.sub.p of a resonant circuit are connected in parallel. The resonant circuit generates output voltages in response to nuclear magnetic resonance magnetization signals received by the receiver antenna L. The amplifier amplifies the output voltages. The output voltages are generated by the nuclear magnetic resonance magnetization signals inducing an EMF in the receiver antenna.
The resonant circuit is an electronically resonant circuit wherein the reactive elements L and C.sub.t store energy. Energy is passed between the reactive elements at resonance while the parallel resistance R.sub.p dissipates energy. The quality factor, Q at the resonant frequency is represented as follows: ##EQU1## where .omega..sub.0 =(L.C.sub.t).sup.-1/2 is the natural resonant angular frequency.
In such a conventional circuit, the voltage signal to noise ratio is proportional to the square root of Q. The Q value represents the number of oscillations before the energy is dissipated in the resonant circuit. In such a conventional resonant circuit, Q must be as high as possible to obtain the best signal to noise ratio. In order to make Q as high as possible, it is desired to keep R.sub.p as high as possible. With good design, a Q of 250 or so is possible.
The conventional circuit as described above is associated with a problem that the bandwidth of the resonant circuit is given by: EQU BW=f.sub.0 /Q (2)
where f.sub.0 is the center frequency; and BW is the full width of the resonant peak at -3 dB.
Therefore, the bandwidth is inversely proportional to Q. There is a phase shift associated with the amplitude variations of the signal from the circuit. At the -3 dB frequencies, there is a phase shift of + or -45.degree. with respect to the phase of the signal at the center frequency. Accordingly, there can be a substantial error, especially in phase shift, over the passband of the circuit. To reduce the amplitude and phase errors, it is desirable to increase the bandwidth of the MRI receiving antenna resonant circuit. Thus, it is desirable to decrease the Q without sacrificing the signal to noise ratio.
In quadrature antenna circuits, where there are two normally orthogonal coils with a small coupling between them, a high Q of conventional circuits magnifies the small coupling between the normally orthogonal coils. Reducing the Q greatly simplifies the coupling and polarization problems associated with quadrature imaging. Accordingly, it is desirable to decrease the Q. In circuits employing either quadrature or surface receiving coils, there is a problem with coupling between the transmitter and receiver resonant circuits. Reducing the Q of the receiver resonant circuit makes decoupling easier. In circuits with multiple receiver coils that are not orthogonal, there is a problem with coupling between the multiple surface receiving coils. Again, reducing the Q of the resonant circuits makes decoupling easier. Thus, it is desirable to reduce the Q of a resonant circuit without degrading the signal to noise ratio.
A conventional method of resistive damping, wherein a damping resistance R.sub.d is placed in parallel with the resonant circuit including the parallel resistance R.sub.p, decreases the Q linearly with the value of R.sub.p but also decreases the signal to noise ratio as a function of the square root of Q.
A Figure of Merit (FOM) comparing the noise resistance R.sub.n to the damping resistance R.sub.d may be defined as follows: ##EQU2##
Conventional resistive damping results in a Figure of Merit of 1 since the same resistor provides the resistances R.sub.n and R.sub.d.
Resistive damping: ##EQU3##
FIG. 1 is a circuit diagram of a conventional feedback circuit for a magnetic resonance imaging or spectroscopy device. The circuit includes a resonant circuit 10, amplifier 18 and a resistive feedback path 20 including a feedback resistor R.sub.f. The resonant circuit 10 includes an MRI receiver antenna 12 illustrated as an inductance L, a capacitor 16 having a capacitance C.sub.t and a parallel resistor 14 with a resistance R.sub.p. The MRI receiver antenna 12, capacitor 16 and parallel resistor 14 are connected in parallel.
The resonant circuit 10 generates output voltages, the voltages across the MRI receiver antenna 12, capacitor 16 and parallel resistor 14, in response to nuclear magnetic resonance magnetization signals received by the MRI receiver antenna 12. The amplifier 18 is coupled across the resonant circuit and amplifies the output voltages. The resistive feedback path 20 is provided from an output of the amplifier 18 to the resonant circuit 10 and carries a feedback current I.sub.f. The amplifier 18 has a negative (inverting) voltage gain of -K and the feedback resistor R.sub.f makes a negative feedback path from the output of the amplifier 18 to the input of the amplifier 18.
For the purpose of analyzing the damping resistance, an effective damping resistance R.sub.d is considered to be added in parallel with the parallel resistance R.sub.p and is represented as follows: EQU (FIG. 1) EQU R.sub.d =R.sub.f /(K+1). (5)
For the purpose of analyzing noise, the noise resistance considered to be added in parallel with the parallel resistor R.sub.p is the value R.sub.n. EQU (FIG. 1) EQU R.sub.n =R.sub.f ( 6)
Accordingly, the Figure of Merit for the conventional feedback circuit of FIG. 1 is: ##EQU4##
Since K is greater than 1, the Figure of Merit for the conventional feedback circuit of FIG. 1 is improved over the Figure of Merit for resistive damping given by equation (4). Nevertheless, it is desired to improve the ratio between the effective noise resistance of the feedback circuit and the effective damping resistance for the resonant circuit even further in order to further decrease the Q without degrading the signal to noise ratio.
In the circuit of FIG. 1, a resistor of a high value, for example, 1 megaohm, is required for the feedback resistor R.sub.f. A resistor of a high value is associated with serious problems of stray and internal capacitance which can cause phase shifts in the feedback and a substantial reduction in the high-frequency resistance of the feedback path. Therefore, it is difficult to obtain a resistor of a high resistance value at high frequencies. In addition, it is desirable to decrease the Q to make decoupling easier for quadrature circuits, and for facilitating the decoupling of quadrature and surface receiving coils in transmitter and receiver resonant circuits as well as multiple surface receiving coils as discussed above.
FIG. 2 is a circuit diagram of a conventional amplifier 22 coupled to a capacitive network 24. The capacitive network 24 includes capacitance 26 and capacitance 28. Capacitance 28 is coupled across the inputs of the amplifier 22. Capacitance 26 is connected in series with capacitance 28. The capacitive network 24 supplies a fraction of the input voltage V.sub.in to the input of the amplifier 22.